A white light source is generally provided by mixing light sources of different wavelength. For example, a conventional white light source can be realized by mixing the red light, green light and blue light with a suitable intensity ratio. Alternatively, the white light source can be realized by mixing the yellow light and blue light with another suitable intensity ratio. The conventional method for manufacturing white light source can be summarized as following:
In a first prior art of white light source, three LED dies based on AlInGaP, GaN and GaP are packaged into a lamp and emit the red light, blue light and green light, respectively. These lights emitted from the lamp can be mixed by a lens to provide a white light.
In a second prior art white light source, two LED dies based on GaN and AlInGaP serve to emit the blue light and yellowish-green light. The blue light and yellowish-green light are mixed together to provide a white light. The white light sources according to above-mentioned two approaches have an efficiency of 20 lm/W.
A third prior art white light source is proposed by Nichia Chemical co., wherein an InGaN based blue LED and a yellow YAG phosphor are used to provide the white light source. This white light source only requires one uni-color LED to provide the white light at the expense of a smaller efficiency of 15 lm/W. However, the phosphor is a mature art and commercially available, thus the cost of manufacturing this kind of white light source is much more cheaper.
A fourth prior art white light source is proposed by Sumitomo Electric Industries Ltd., which uses a white-light LED based on ZnSe. A CdZnSe thin film is formed on the surface of a ZnSe crystalline substrate and serves to emit the blue light. The ZnSe crystalline substrate emits the yellow light after receiving the blue light of the CdZnSe thin film. The blue light and the yellow light are mixed to provide a white light. In this reference, only one LED chip is required and the operation voltage thereof is 2.7 V, smaller than the 3.5 V operation voltage of the GaN based LED. Moreover, no phosphor is required.
In a fifth approach to provide a white light source, an ultra-violet LED is used to excite a plurality of phosphors such that the phosphors are able to emit lights of different colors for the sake of generating a white light. The phosphors are prepared by following processes:
1. Synthesizing a phosphor powder with a formula of Y2O3:Eu such as (Y1.9Eu0.1)O3 by the solid-state reaction or chemosynthesis method such as citrate sol gel, co-precipitation or micro emulsion method.
2. Synthesizing a phosphor powder with a formula of SrAl2O4:Eu such as (Sr0.96Eu0.04)Al2O4 by the solid-state reaction or chemosynthesis method such as citrate sol gel, co-precipitation or micro emulsion method.
3. Synthesizing a phosphor powder with formula of BaMgAl10O17: Eu, Mn such as (Ba0.9Eu0.1)(Mg0.96Mn0.04) Al10O17 by the solid-state reaction or chemosynthesis method such as citrate sol gel, co-precipitation or micro emulsion method.
4. Measuring the emission spectrum of the above-mentioned three phosphor powders mixed in a predetermined ratio and excited by a 396 nm ultraviolet light. As shown in FIG. 3, the phosphor powder after being excited by a 396 nm ultraviolet light emits a light and color of the light is coordinated with reference to the chromaticity diagram by CIE (commission internationale del'Eclairage) in 1931. The light is marked at point A of the chromaticity diagram, as shown in FIG. 3.
However, light of the light source formed by the combination of three phosphor powders is not as ideal as the natural sun light in the aspects of wavelength distribution and continuousness. The white light thus produced has a relatively poor chroma, which is, even indistinguishable to human eyes, differentiable to the optically sophisticated instrument such as cameras. Therefore, the color rendering property and reproducing ability of this white light source are not satisfactory and it is used mainly for the lighting under some general circumstance.